The present invention relates generally to zooming systems, and more specifically to a zooming system which is arranged to control the moving speed of a zoom lens thereof in directions along its optical axis. This invention may be applicable particularly, but not exclusively, to video cameras.
A video camera having a zoom lens system as shown in FIG. 1, is known and comprises four groups of lens systems, i.e., a fixed convex lens system L1, a movable concave lens system L2, a movable convex lens system L3 and a fixed convex lens system L4. Image-information light l from an object to be taken at infinity focus, for example, is incident upon the fixed convex lens system L1 in a parallel state and then refracted so as to be focused on the second focal point F1 of the fixed convex lens system L1 and further refracted by means of the movable concave lens system L2 so as to be equivalent to a light beam emitted from the first focal point F3 of the movable convex lens system L3. The refraction light l due to the movable concave lens system L2 is collimated by means of the movable convex lens system L3 and then focused by means of the fixed convex lens system L4 to be image-formed on an image pickup surface 1 positioned to correspond to the second focal point F4 of the fixed convex lens system L4. Here, the focal lengths of the lens systems L1 to L4 are respectively illustrated at characters f1 to f4 and the focal length of this zoom lens system is represented by a character f0. In such a zoom lens system, the movable convex lens system L3 is movable in accordance with movement of the movable concave lens system L2 in the directions of its optical axis as indicated by reference character B in FIG. 1 so as to keep the formation position of an image corresponding to the object taken at infinity focus, resulting in control of the focal length f0 thereof, i.e., zooming. Here, let it be assumed that the distance between the second focal point F1 of the fixed convex lens system L1 and the movable concave lens system L2 is a and the distance between the first focal point F3 of the movable convex lens system L3 and the movable concave lens system L2 is b, the relationship in position between the movable concave lens system L2 and the movable convex lens system L3 is determined so as to satisfy the following equation (1): ##EQU1## That is, in response to movement of the movable concave lens system L2. i.e., a change of the distance a therebetween, the movable convex lens system L3 is moved to change the distance b therebetween, thereby satisfying the above-mentioned equation (1). On the other hand, this zoom lens system is generally arranged such that the movements of the lens systems L2 and L3 in the optical-axis directions is effected by means of a cylindrical cam mechanism and the distance a and the angle .theta. of rotation of the cylindrical cam are in a linear relationship to each other as shown in FIG. 2. However, an important problem in such a zoom lens system relates to the fact that the focal length f0 thereof is not in a linear relation to the angle .theta. of rotation of the cylindrical cam and is abruptly varied with respect to the angle .theta. of rotation thereof, particularly, when the movable concave lens system L2 is moving toward the longer focus side thereof. This results in abrupt variation of the size of the image with respect to the same object, whereby the image becomes unnatural.
Furthermore, in such a zoom lens system, the focusing for an object positioned at finite focus is effected by movement of the fixed convex lens system L1 or L4 so that the image-formation point A is kept on the image-pickup surface 1. There is a problem which arises with this type of zoom lens system, however, in that the position of the image-formation point A changed due to the variation of the focal length f0 of this zoom lens system is not in a linear relationship to the angle .theta. of rotation of the cylindrical cam and abruptly varies with respect to the variation of the angle .theta. of rotation thereof, particularly, when the movable concave lens system L2 is moving toward its longer focus side. The position of the image-formation point A reaches the maximum when the object to be taken is at close range. Therefore, in the case of the focusing made by the fixed convex lens system L4 (rear lens system) when the object is at finite focus, particularly at close range, the abrupt variation of the position of the image-formation point A results in the fact that difficulty is encountered in terms of the follow-up of the fixed convex lens system L4, thereby causing out-of-focus of the image on zooming.